<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>webgl着色器</title>
  <script src="lib/webgl-debug.js"></script>
  <script src="lib/webgl-utils.js"></script>
  <script src="lib/cuon-utils.js"></script>
  <script src="lib/cuon-matrix.js"></script>
</head>

<body onload="main()">
  <canvas id="canvas" width="400" height="490"></canvas>
</body>
<script>
  // 顶点着色器伪代码
  var vShder_source = `
  attribute vec4 a_Position;
void main(){
gl_Position =a_Position; //设置坐标
gl_PointSize =10.0; //设置尺寸
}
`
  //片元着色器伪代码
  var fShder_source = `
precision mediump float ;
uniform vec4 u_FragColor;
void main(){
gl_FragColor = u_FragColor; //设置颜色
}
`
  function main() {
    var canvas = document.getElementById('canvas');
    //  获取上下文对象
    var gl = getWebGLContext(canvas);
    //  初始化着色器
    if (!initShaders(gl, vShder_source, fShder_source)) {
      return;
    }
    // 指定一个覆盖canvas的颜色
    gl.clearColor(1.0, 1.0, 1.0, 1.0);
    // 清除指定的缓冲区
    gl.clear(gl.COLOR_BUFFER_BIT);

    //获取着色器变量的储存位置
    var a_Position = gl.getAttribLocation(gl.program, "a_Position");
    var u_FragColor = gl.getUniformLocation(gl.program, "u_FragColor");

    gl.clearColor(0.0, 0.0, 0.0, 1.0);
    gl.clear(gl.COLOR_BUFFER_BIT);
    canvas.onmousedown = function (event) {
      click(event, gl, canvas, a_Position, u_FragColor);
    }

  }

  var g_point = [];
  function click(event, gl, canvas, a_Position, u_FragColor) {
    var e = event || window.event;
    var x = e.clientX;
    var y = e.clientY;
    var rect = e.target.getBoundingClientRect();
    x = ((x - rect.left) - canvas.width / 2) / (canvas.width / 2);
    y = (y - rect.top - canvas.height / 2) / (canvas.height / 2) * -1;

    g_point.push({ x: x, y: y });
    gl.clear(gl.COLOR_BUFFER_BIT);
    for (var i = 0; i < g_point.length; i++) {
      var item = g_point[i];
      gl.vertexAttrib3f(a_Position, item.x, item.y, 0);
      var color = { r: Math.random(), g: Math.random(), b: Math.random(), a: 1 };
      gl.uniform4f(u_FragColor, color.r, color.g, color.b, color.a);
      gl.drawArrays(gl.POINTS, 0, 1);
    }
  }

</script>

</html>